This invention pertains to semiconductor integrated circuits and their fabrication. More particularly, a junction and contact having a diffusion barrier to control silicidation of a substrate is provided.
The circuit elements of integrated circuits continue to decrease in size. Many processing operations are included in the manufacture of modem circuits. One of the common manufacturing processes is the deposition of metal or other conductive materials as a gate, interconnect or contact material. One of the requirements of conductive contact materials is that they withstand heating to a few hundred degrees while in contact with silicon. Under these conditions, many metal-silicon contacts cannot exist because the silicon and the metal react to form a silicide. The suicides generally have high electrical conductivity and in themselves make dependable and self-aligned silicon contacts, however. See, for example, Semiconductor Integrated Circuit Processing Technology, Addison-Wesley Publishing Co., 1990, pp. 535-549.
Therefore, suicides are used in place of metals in many circuits.
Another commonly used wafer fabrication operation is diffusion of dopants into a substrate. The first step is to deposit the dopant material on the substrate surface. The second step is to xe2x80x9cdrivexe2x80x9d the dopant into the substrate by exposing the coated substrate to a selected high temperature for a selected time so as to achieve the desired diffusion depth. In this diffusion process, with the advent of increasingly large-scale integrated manufacturing, the need for ultra-shallow junctions in silicon has arisen. There is also a need to integrate this doping step with the formation of a contact. When a silicide is used to form the contact, there is need to form a diffusion barrier between the contact and the substrate, so as to control silicidation in the substrate material. It is known to use diffusion barriers to prevent undesirable materials from reaching parts of the circuit where a harmful effect results. Barriers have been categorized as passive or sacrificial. Passive layers are inert to the material on each side of the layer and thus will keep the two layers separated if they are completely effective. Often there is substantial diffusion of one or both components through the passive barrier, however. See Semiconductor Integrated Circuit Processing Technology, pp. 550-551.
Although many diffusion barriers are known, a better diffusion barrier to avoid silicidation of a silicon substrate having a silicide contact is needed.
A junction in a silicon substrate is provided along with a silicide contact. A conductive compound formed at the interface between the substrate and the contact is used to prevent silicidation in the substrate to the extent that the junction is decreased in effectiveness. A layer of a dopant, such as boron, is initially applied to the surface of the substrate by evaporation. Sufficient boron is applied such that residual boron is available on the surface of the substrate after a junction is formed in the silicon by diffusion. Then a cap of amorphous silicon is deposited over the boron by evaporation. At this point, two procedures may be used to form the junction and contact of this invention. In one embodiment, rapid thermal processing is used to form the junction by diffusion, then a titanium layer is deposited on the substrate and a silicon cap is then applied and rapid thermal processing is again used to form the final junction and contact. In another embodiment, after the first silicon layer is formed by evaporation above the boron layer, a titanium layer is formed, followed by a silicon layer and then rapid thermal processing is used to form the junction and contact at the same time. A preferred metal for forming the silicide is titanium, but other metals known to form a silicide and which form a compound with the dopant under the conditions of rapid thermal processing, such as tungsten, may be used. These metals that have lower resistivity than their suicides can also be deposited directly on the boron layer without the sacrificial silicon layer in order to form a metal contact instead of the suicides.